Brushless DC motor, hereinafter also referred to as a brushless motor or a motor, is typically driven by AC (Alternating Current) power obtained by converting DC (Direct Current) power through inverter circuit. The voltage of AC power obtained from the inverter circuit is pulsed. In order to keep the brushless motor running, conduction of armature winding needs to be switched from one phase winding to another at a certain rotational position of the rotor, in other words, at a certain timing. This timing of conduction switching is known as commutation timing of winding current. The commutation timing of winding current is correlated with the rotational position of the rotor through control of the ON/OFF drive of switching elements in the inverter circuit. This correlation is typically established through a detecting element that detects the rotational position of the rotor. Examples of such detecting elements include a Hall sensor that detects the actual rotational position of the rotor and a current detector that detects the rotational position of the rotor indirectly through a phase current of the phase winding.
In order to detect multiple phases of phase current by a single current detector, the current detector is disposed on the DC side of the inverter circuit. Examples of current detector include a Hall element type current detector that detects count of magnetic flux generated by DC current and a resistor type current detector that detects a current value as a voltage value. The current detectors, when used in speed control or torque control of the brushless motor, may detect load current the detection signal of which may be used as a feedback signal of the respective controls. In such applications, a current detector may comprise a combination of a detector used in a feedback control and a Hall sensor used in detection of rotor position.
Precise detection of phase current is desired for these current detectors to achieve accurate and stable commutation timing. However, because voltage occurring at the two ends of current detecting resistor in response to the passing current is extremely small, precision of current detection is easily affected by unfavorable properties specific to inner circuitry such as amplifiers and temperatures of peripheral elements and environment. It is specifically desired to reduce the impact of offset voltage/current produced by the properties of detection circuitry such as amplifiers.